A dehydrogenation reaction of a hydrogen-containing organic compound is one of the most important reactions in organic synthesis, and is a reaction that has high utility value in industry. For example, a conversion reaction involving a dehydrogenation reaction (oxidation reaction) of an alcohol into a carbonyl compound such as an aldehyde, a ketone, or a carboxylic acid plays an important role in the production of organic compounds used in many fields such as pharmaceuticals, agrochemicals, food, fragrances, and materials, or starting materials therefor. Furthermore, a method for producing hydrogen by a dehydrogenation reaction of an alcohol, formic acid, or a formate is a technique that has been attracting attention as a technique for supplying and storing hydrogen for a fuel cell.
Synthesis of a carbonyl compound by an oxidative dehydrogenation reaction of an alcohol is one of the most important functional group conversions in organic syntheses for obtaining intermediates for pharmaceuticals, agrochemicals, fragrances, etc., and in the past a large number of excellent oxidizing agents and oxidation reactions have been developed. For example, as a stoichiometric oxidizing reagent, an oxidation method using a heavy metal oxidizing agent (potassium permanganate, bichromic acid or a salt thereof, chromium trioxide, etc.), a DMSO oxidation method (Swern oxidation, etc.), etc. are known. It is difficult to use these oxidizing agents and oxidation methods industrially in terms of safety and environmental friendliness due to the production of large amounts of highly toxic waste material by-products and the occurrence of bad odors.
On the other hand, from the viewpoint of environmentally friendly green chemistry, catalytic oxidation reactions of alcohols using a co-oxidizing agent such as hydrogen peroxide, acetone, or molecular oxygen have been developed. However, a reaction using a co-oxidizing agent has the problems that depending on the type of co-oxidizing agent, the reaction becomes complicated, and the substrates to which it can be applied are limited, and there is still room for improvement from the viewpoint of design of a catalytic reaction based on atom efficiency.
Thus, from a process chemistry viewpoint it is very important to develop a catalytic oxidation reaction of an alcohol without using a co-oxidizing agent, that is, a catalytic oxidative dehydrogenation reaction. In recent years, such reactions have been reported one after another and, for example, oxidative dehydrogenation reactions using ruthenium catalysts (Non-Patent Documents 1 to 5) or iridium catalysts (Non-Patent Documents 6 and 7) have been reported. However, there are problems in terms of industrial application, in that these reactions are carried out at a relatively high temperature, a reaction requiring basic conditions cannot be applied to a substrate that is unstable toward a base, and the amount of catalyst is relatively large.
Furthermore, Non-Patent Document 8 reports a dehydrogenative oxidation reaction of an alcohol using a cationic iridium complex; the reaction is carried out in aqueous solvent under reflux conditions, but while taking into consideration safety and economy it is desirable for it to be carried out at a lower temperature. Moreover, in this reaction, since the cationic iridium complex itself exhibits acidity, it is not suitable for a reaction of a substrate that is susceptible to decomposition in an acidic state. Furthermore, Non-Patent Document 9 reports a catalytic dehydrogenation reaction of a cyclic amine (2,6-dimethyldecahydro-1,5-naphthyridine) using a neutral iridium complex, but the use of an alcohol in a catalytic dehydrogenation reaction has not been tried.
From the above, there has been a desire for the development of a catalytic dehydrogenation reaction of an alcohol that can be carried out with a small amount of catalyst at a relatively low temperature.
Furthermore, an aldehyde is prepared by an oxidation reaction of a primary alcohol, and the aldehyde is further oxidized to give the corresponding carboxylic acid; being able to make this proceed as a one pot reaction is very important from the viewpoint of process chemistry. As a stoichiometric oxidizing agent that can be used for this purpose, potassium permanganate (KMnO4), Jones reagent, and pyridinium dichromate (PDC) are known, but it is difficult to carry out these methods industrially in terms of economy and safety such as a large amount of heavy metal being used and a highly toxic compound being produced as a by-product.
As catalytic methods, oxidation methods using ruthenium tetroxide, and TEMPO (Non-Patent Document 10) are known, but due to the conditions being relatively severe it is difficult to apply them to compounds having a large number of functional groups, and due to the use of a co-oxidizing agent there is still room for improvement from the viewpoint of design of a catalytic reaction based on atom efficiency. A method using sodium tungstate (Non-Patent Document 11) as a catalyst is a reaction that is accompanied by danger due to the use of a high concentration of aqueous hydrogen peroxide. 1-Me-AZADO oxidation (Patent Document 1), which has improved the defect of TEMPO oxidation, has also been developed, but since this method also requires a large amount of co-oxidizing agent, there has been a desire for the development of a catalyst that can reduce the environmental burden.
As described above, there has been a desire for the development of an oxidative dehydrogenation reaction that will take a primary alcohol to a carboxylic acid via an aldehyde and that will progress with a small amount of catalyst without using a co-oxidizing agent.
On the other hand, hydrogen (H2) has conventionally been utilized in various industrial fields, such as for petroleum purification or as a chemical starting material, and in recent years it has received attention as fuel for a fuel cell. However, since hydrogen is gaseous at room temperature, highly reactive, and susceptible to ignition in air, the stable supply and storage of hydrogen is an important issue in the development of fuel cells. For example, as methods for storing hydrogen there are known a method in which it is stored as a compressed gas, a method in which hydrogen gas is liquefied and stored in the form of liquid hydrogen, and a method in which hydrogen is taken into a hydrogen absorbing alloy and stored. However, these methods have the problem that the amount of hydrogen stored per unit weight of storage medium is small and, in addition, there are problems with cost, safety, and handling.
In order to solve these problems, a method for storing hydrogen in the form of a substance other than H2 could be considered. For example, formic acid (HCO2H) is known to generate hydrogen (H2) and carbon dioxide (CO2) when strongly heated. It is possible by utilizing this to store hydrogen in the form of formic acid, which is a stable substance, and to stably supply hydrogen by appropriately heating formic acid and generating hydrogen. However, since it is necessary to carry out a thermal decomposition reaction of formic acid at a high temperature, there has been a desire for the development of a catalyst that can generate hydrogen from formic acid with high efficiency under mild conditions.
As catalysts for the decomposition of formic acid, examples using a metal complex have already been reported. For example, a polynuclear metal complex containing iridium and ruthenium has been reported in Patent Document 2, but due to the use of two types of transition metals the production cost is high. Furthermore, a decomposition reaction of formic acid using a rhodium complex has been reported in Patent Document 3, but the rhodium complexes in the examples are limited to cationic aquo complexes having a bipyridyl-based ligand, and the amount of catalyst used is about 1 mol %, which cannot necessarily be said to be efficient.
From the above, there has been a desire for the development of a catalyst for a decomposition reaction of formic acid or a formate that has high reactivity with a small amount of catalyst under mild conditions.